Session Item

Stereotactic ablative body radiotherapy (SABR) is an effective treatment for patients with prostate cancer. SABR prostate treatment has been carefully implemented using standard linear accelerators however, the advanced imaging, recontouring and plan re-optimisation workflow of the MR-Linac lends itself to SABR prostate treatment. This study assessed the feasibility of magnetic-resonance guided adaptive radiotherapy (MRgART) for prostate cancer. 

Fifteen patients with localised prostate cancer were treated with stereotactic prostate MRgART. Plans were prescribed to 36.25Gy to Planning Target Volume (PTV) and Clinical Target Volume (CTV) boosted to 38-40Gy in 5-6 fractions with the goal of 95% PTV to receive 100% of the prescription. OAR constraints were as per published guidelines. All fractions were delivered using an adapt to shape (ATS) workflow where the MRI is recountoured to create a new plan daily. Treatment times were recorded daily. Acute toxicity with a focus on genitourinary (GU) and gastrointestinal (GI) symptoms were measured for up to 3 months after treatment, using Common Terminology Criteria for Adverse Events (CTCAE) v.5.0. 

A total of 76 fractions (adapted plans) were successfully delivered to 15 patients. Of these patients, one had SpaceOar inserted and received 6 fractions. The median patient age was 73 years (range: 61-87 years). Median total treatment time including patient set-up, imaging, contouring, plan adaptation and beam-on was 50.2 minutes (range 43.2-63.7 minutes), and median beam-on only time was 14.1 minutes (range 7.3-16.8 minutes). 3-month follow-up data were complete for 13 patients. Four patients reported grade 1 GU toxicity, and no grade 2 or worse early GU toxicity was reported. The vast majority of GU toxicity was reported for pain, frequency and urgency. One patient reported grade 1 GI toxicity (diarrhoea) and another patient reported grade 3 GI toxicity (rectal pain).   

Our experience with SABR prostate using MRgART has demonstrated safety and feasibility for this group of patients. The preliminary results gained from assessing the low incidence of GI and GU toxicity indicates that it is possible to safely deliver a stereotactic highly conformal dose to the prostate with confidence. We continue to collect follow-up data to assess late toxicity and patient reported outcome.